a. Field of the Invention
The present invention relates to a noise-cancelling apparatus in an FM stereo receiver, and more particularly it pertains to a noise-cancelling apparatus used in an FM stereo receiver, which is arranged to be operative so that, whenever the level of the signal received drops below a predetermined level, the signal of the left channel is adapted to be blended with a certain volume of the signal of the right channel substantially evenly substantially throughout the entire range of audio frequency on the part of the signal receiver and vice versa at the same time, so that the signal to noise ratio, S/N ratio, is improved.
B. Brief Description of the Prior Art
It is known that FM broadcast is not comparable to AM broadcast in that the signal to noise ratio, S/N ratio, of the former is much superior to that of the latter. However, the FM stereo broadcast is susceptible to being affected by noise components as compared with FM monoral broadcast. For this reason, in case FM stereo signals are received by an FM stereo receiver, the noise components in the high frequency range will tend to increase if the level of signals received drops to a low value, causing the S/N ratio to aggravate.
In order to prevent aggravation of the S/N ratio, there has been proposed in the past an FM stereo receiver which is arranged so that a high-cut filter (a filter designed to eliminate frequencies above a particular level) is adapted to be connected to or disconnected from the signal line by a switching means. Such an FM stereo receiver, however, is designed so as to cut not only the noise components in the high frequency range but also the signals received as well over the band of 3 to 10 kHz. Accordingly, the known FM stereo receiver has the drawback that the quality of the program source is deteriorated.
Also, as a means for improving the aforesaid S/N ratio, there is known a noise-cancelling apparatus for use in an FM stereo receiver, which is arranged so that the high frequency range components in the signals of the right and left channels are blended with each other via a capacitor. In this known noise-cancelling apparatus, however, the amount of signals of the right and the left channels which are blended with each other will increase greater for a higher frequency range. Thus, there arises the unavoidable tendency that the separation of channels becomes poorer for higher frequency range, and that the localization of sound image will fluctuate depending on the frequency of the signal received, giving a sense of loss of naturalness to the listener. Furthermore, in this known noise-cancelling apparatus, the inverted phase components in the signals of the right and the left channels will be cancelled with each other more intensively for higher frequency range. Thus, there is the drawback that the frequency characteristics will be heard as decaying greater for a higher frequency range.
Moreover, the aforesaid high-cut filter and the noise-cancelling apparatus of the known art are usually arranged so that such device is adapted to be automatically connected to and disconnected from an FM stereo receiver by a switching means which is capable of being actuated in accordance with the level of the signals received by the FM stereo receiver. Thus, there is the need for the provision of means for obtaining a control signal for controlling said switching means. With the conventional means for obtaining said control signal, there has been the inconvenience that in case there is a divergence in the gain of a radio-frequency amplifier or an intermediate frequency amplifier due to manufacturing error, there will arise a discrepancy in the level of the control signal obtained relative to the signal received of the same level. Thus, it has been necessary to adjust the levels of the control signal for the respective FM stereo receivers manufactured. This adjusting operation requires a great deal of labor, thus constituting a cause for an increase in the manufacturing cost.